Because of their high degree of nonstickiness and high chemical resistance, fluorine resins have been used extensively in cooking utensils (e.g. frypans, pots, cooking plates, the inner container of jar type rice cookers, etc.) and in such industrial fields as the food industry, electric industry and mechanical industry.
A common method for coating metal (e.g., aluminum) substrates with fluorine resins consists of roughening the surface of the substrate such as by blasting, applying an undercoat (primer) to the substrate and depositing a topcoat of a fluorine resin which will adhere strongly to the metal substrate by the bonding action of the primer coating (the primer method as described, e.g., in JP-B-62-5466). (The term "JP-B" as used herein means an "examined Japanese patent publication").
Alternatively, tiny cavities resembling octopus traps are formed in the surface of the substrate and a coating of fluorine resin is applied so that it will adhere mechanically to the substrate through the "anchor effect" of the cavities (the etching method as described, e.g., in JP-B-56-35516).
The so prepared fluorine resin coated articles have such low resistance to wear by metals that they are not ideal for use in applications that require high wear resistance such as frypans, pots and cooking plates. To deal with this problem, several proposals have been made, including: coating with a mixture of a fluorine resin with ceramic powder and/or metallic powder; thermal spraying ceramic powder and/or metallic powder onto an aluminum substrate to form a hard coating on the aluminum surface and then coating it with a fluorine resin; and roughening the surface of an aluminum substrate by etching, forming a hard coating on the roughened surface by anodization, and coating it with a fluorine resin.
These methods, however, have proved to be still ineffective in providing fluorine resin coated articles with high wear resistance at elevated temperatures (hereinafter sometimes referred to as "hot wear resistance").